System design and cooling method for LP steam turbines using last stage hybrid bucket

ABSTRACT

A steam turbine system for use in conjunction with hybrid last stage(s) LP buckets. The system is adapted to cool the bucket tip region during low VAN windage conditions whereby the beneficial design and efficiency outcomes of the use of hybrid blades can be realized.

BACKGROUND OF THE INVENTION

This invention relates to a steam turbine system design to be used in conjunction with hybrid composite buckets (blades) in the last stage(s) of a steam turbine, typically a low pressure (LP) steam turbine section.

Steam turbine blades operate in an environment where they are subject to high centrifugal loads and vibratory stresses. Vibratory stresses increase when blade natural frequencies become in resonance with running speed or other passing frequencies (upstream bucket or nozzle count, or other major per/rev features). The magnitude of vibratory stresses when a blade vibrates in resonance is proportional to the amount of damping present in the system (damping is comprised of material, aerodynamic and mechanical components, as well as the vibration stimulus level). For continuously coupled blades, the frequency of vibration is a function of the entire system of blades in a row, and not necessarily that of individual blades within the row.

Furthermore, for turbine buckets or blades, centrifugal loads are a function of the operating speed, the mass of the blade, and the radius from engine centerline where that mass is located. As the mass of the blade increases, the physical area or cross-sectional area must increase at lower radial heights to be able to carry the mass above it without exceeding the allowable stresses for the given material. This increasing section area of the blade at lower spans contributes to excessive flow blockage at the root and thus lower performance. The weight of the blade contributes to higher disk stresses and thus to potentially reduced reliability.

Several prior U.S. patents/applications relate to so-called “hybrid” blade designs where the weight of the airfoil is reduced by composing the airfoil as a combination of a metal and polymer filler material. Specifically, one or more pockets are formed in the airfoil portion and filled with the polymer filler material. These prior patents/applications include U.S. Pat. Nos. 6,854,959; 6,364,616; 6,139,278; 6,042,338; 5,931,641 and 5,720,597; application Ser. No. 10/900,222 filed Jul. 28, 2004 and application Ser. No. 10/913,407 filed Aug. 7, 2004; the disclosures of each of which are incorporated herein by this reference.

Another issue relating to the use of hybrid steam turbine blades, however, relates to cost as a function of temperatures experienced by such blades during use. In a double flow steam turbine, for example, there is significant windage heating of the last stage blade tip area during partial load and full speed conditions. The hood area behind the blades has a water spray system to cool the exhaust flow to the condenser. In this regard, in a typical design the water sprays are not used to cool the bucket but to cool the exhaust steam to keep the exhaust casing seal within its material temperature limits. Typically this is a urethane seal between the LP hood and the condenser opening. Even during the operation of the water sprays, the cooling flow does not migrate to the heated area near the blade tips and, thus, cooling of the blade tips is minimal. The blade tips during this condition can reach in excess of 500° F. whereas, during normal operation, the blade temperatures reach only about 150° F.

BRIEF DESCRIPTION OF THE INVENTION

This invention expands the hybrid blade concept by providing a steam turbine system designed for use in conjunction with hybrid last stage(s) LP buckets. In particular, the present invention provides a system that can cool the bucket tip region during low VAN windage conditions whereby the beneficial design and efficiency outcomes of the use of hybrid blades can be realized.

Thus, the invention may be embodied in an axial flow steam turbine including: a rotor; a last stage including a diaphragm comprising an inner ring, an outer ring, and a plurality of nozzles extending therebetween, and a row of buckets secured to said rotor downstream of said diaphragm for rotation with respect to said last stage diaphragm; and at least one injection assembly for injecting cooling media toward a vicinity of said last stage.

The invention may also be embodied in a method of cooling the last stage of an axial flow steam turbine including a rotor, the last stage including a diaphragm comprising an inner ring, an outer ring, and a plurality of nozzles extending therebetween, and a row of buckets secured to said rotor downstream of said diaphragm for rotation with respect to said last stage diaphragm, the method comprising: injecting cooling media toward a vicinity of said last stage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a double-flow low pressure turbine;

FIG. 2 is a schematic perspective view of a partially completed hybrid blade;

FIG. 3 is a schematic side elevation of a turbine wheel having a plurality of hybrid blades mounted thereon;

FIG. 4 is a cross-sectional view of a hybrid blade;

FIG. 5 is a schematic illustration of a hybrid bucket system incorporating steam or water injection and/or water sprays in an example embodiment of the invention;

FIG. 6 is a schematic elevational view of steam injection through the nozzle forward outer side wall according to an example embodiment of the invention;

FIG. 7 is a schematic side elevational view illustrating a steam or water injection flow circuit in an example embodiment of the invention; and

FIG. 8 is a schematic illustration of a steam or water injection/moisture extraction assembly

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic diagram of a double-flow, low pressure turbine 10 including a turbine casing 12, rotor 14 and a plurality of wheels in two turbine sections indicated at 16, 18. The areas 20, 22 circled in dotted lines represent the radially outermost regions of the last stage blades that have been shown to experience the most windage heating during partial load conditions. Thus, in example embodiments of the invention, steam or water injection, steam extraction, and/or water sprays are used to cool the nozzles and/or buckets of the last stage(s), so that the advantages of hybrid blades for such stage(s) can be realized.

FIG. 2 schematically shows an example construction of a hybrid steam turbine blade 24. The steam turbine blade includes a shank portion 26 and an airfoil portion 28. The airfoil portion has an operating temperature range, a design rotational speed, a blade root attached to the shank portion, a blade tip, and a radial axis extending outward toward the blade tip and inward toward the blade root. The shank portion typically includes a dovetail for attachment of the blade to a rotor disc (FIG. 3), and a platform for helping to radially contain the steam flow. The airfoil portion has a leading edge and a trailing edge, with the steam flow direction generally being from the leading edge to the trailing edge. The airfoil also has a pressure side and suction (convex) side.

In the illustrated example, radially inner and outer pockets 30, 32 are formed on the pressure side of the airfoil portion 28, separated by a relatively wide web or rib and a mid-span damper 36. More (or fewer) pockets can be included in the blade design. FIG. 3 illustrates schematically a row of hybrid blades 24, mounted on a turbine rotor wheel 42.

The airfoil includes a main body or section 34 consisting essentially of metal. In this regard, the term “metal” includes “alloy” but for the purposes of describing the invention herein is not considered to mean a “metallic foam”. In the example embodiment described herein, the main body 34 is a monolithic metallic section, although the invention is not necessarily limited in this regard. The metallic section has a first mass density and radially extends generally from the blade root to the blade tip. The pockets or recesses 30,32 are defined in the airfoil where the metal is omitted or removed. In this regard, the main body or metallic section of the blade is forged, extruded or cast and the surface recesses may be formed by machining such as, for example, by chemical milling, electrochemical machining, electro-discharge machining or high speed machining.

FIG. 4 is a cross-sectional depiction of the hybrid blade structure wherein a filler section 40 that does not consist essentially of metal and that has a second mass density, different from the first mass density, is provided in pocket 30 of the metal section. The filler material 40, 38 for pockets 30, 32 may comprise urethane-based polymers of different durometer, silicone-based polymers, rubber-based compounds or polymer mixtures with suitable stiffeners and/or other materials such as carbon fibers, glass fibers or ceramics to adjust frequency, damping, erosion-resistance, etc. Some suitable filler compositions are disclosed, for example, in U.S. Pat. Nos. 6,287,080 and 5,931,641, the disclosures of which are incorporated herein by this reference. Choices for bonding the filler material 38,40 to the metal surface of the airfoil portion include, without limitation, self adhesion, adhesion between the filler material and the metal surface of the airfoil portion, adhesive bonding (adhesive film or paste), and fusion bonding.

If deemed necessary or desirable, the filler material 38 used to fill pocket 32 may have different properties, such as temperature resistance, as compared to filler material 40 used to fill pocket 30. The utilization of different filler sections, or more specifically filler materials, permits improved temperature capability of hybrid blades at reduced cost. Each material used could be formulated for specific locations on the bucket based on temperature characteristics of the filler materials and temperature capability requirements of the blades in any given stage. Using the more expensive, high temperature, materials in a limited location on the bucket makes the design of hybrid blades more feasible especially for those blades that experience high windage conditions.

The blades may be manufactured with one or more pockets filled with filler materials chosen to achieve the desired natural frequencies of the individual blades as well as the entire row of blades.

In a first method associated with this example embodiment, the pockets 30, 32 of blades 24 within a row of such blades are filled with filler materials chosen as a function of natural frequency. Thus, all of the pockets (from one to four or more) could be filled with a similar polymer filler material designed to achieve the desired natural frequencies of the individual blades as well as the entire row of blades. In another example, each blade would incorporate at least two different filler materials of, for example, different stiffness, to achieve the desired natural frequencies.

In a second method associated with this example embodiment, two or more groups of blades with recessed pocket(s) along the pressure side of the airfoil may be formed with different filler materials in the pockets of the blades of each group. By way of example, one group of blades may use a higher strength or “stiffer” material as the pocket filler, while the other group of blades may use a lower stiffness material. Alternatively, plural pockets in the blades of one group may be filled with plural polymer fillers, respectively, and the plural pockets of the other group may be filled with respectively different plural polymer fillers. Thus, for example, and with reference to blade in FIG. 2, pocket 30 may be filled with polymer “a” and pocket 32 filled with polymer “b” for a first group of blades. For a second group of blades, pocket 30 may be filled with polymer “c” and pocket 32 filled with polymer “d.” Again, these materials are chosen so as to achieve different resonance frequencies in the two groups of blades.

The blade designs described above may be utilized to form a row of blades on a steam turbine rotor wheel as illustrated in FIG. 3. Specifically, groups A and B, may be assembled on the turbine wheel in a predetermined mapped configuration for example, in the pattern ABAB . . . , such that a blade of group A is always adjacent a blade of group B. In this way, the two (or more) populations of blades maybe purposefully manufactured and logically assembled so as to utilize their inherent differences in resonance frequencies as a means of reducing the system response to synchronous and non-synchronous vibrations, without adversely affecting the aerodynamic properties of the blade design. Further in this regard, there exists the potential to design one group of blades where the natural frequency is equally disposed between two “per-rev” criteria (4 per rev and 5 per rev split for example), and to design the other group of blades with a different filler material, so as to be equally disposed about another set of “per-rev” stimuli (such as a 3 per rev and 4 per rev split).

It is also possible to vary the pattern of blade group distribution, again so as to achieve the desired frequency characteristics. For example, a pattern AABBAA . . . or AABAAB . . . might also be employed.

In another example embodiment, the blades are manufactured with one or more pockets filled with urethane or silicon polymer filler materials chosen as a function of damping characteristics of the filler materials.

Again this may be accomplished in one of two methods. The first method would be to use one or more multiple fillers within the pockets of each blade (or pockets of blade), chosen to alter the damping coefficients of each of the blades as well as the damping response of the entire row of blades. Depending upon where the specific material properties are required, some pockets could be filled with either a highly damped material or a material that may meet some other specific requirement, not necessarily related to damping. In some areas of the blade, for example, erosion may be a concern; materials that are desirable for erosion prevention, however, may not be desirable for vibration reduction. In other areas, erosion may not be as much of an issue, and vibration damping may be the principal concern. In any event, by altering the damping characteristics to a greater or lesser extent, the magnitude of the system vibrations in the row of blades may be reduced to a tolerable level.

The second method associated with this example again involves the separation of the blades into two discrete groups, each of which incorporates different filler materials to adjust the damping coefficient of the blades within the respective groups. For example, all of the blades of one group would incorporate one or more fillers in the respective pockets, while all of the blades of the second group would incorporate a different choice of one or more fillers. The blades would be assembled in a mapped configuration like those described above, i.e., ABAB . . . or AABBAA . . . , etc. The mapped configuration results in mixed tuning of the set of blades via various damping responses of the blades in each group of blades to create a more damped blade row or set. This may also shift the frequencies of each blade to take even greater advantage of the mixed tuning concept.

Each of the above methods may lead to the removal of the typical mechanical damper at the mid-span of certain blade designs. This mid-span connection is a flow disturbance that leads to reduced turbine efficiency. In other words, by using appropriate filler materials with improved damping properties, the complete removal of the current mid-span damper 36 is possible.

As noted above, a typical hybrid bucket 24 consists of a metallic blade section 34 with a recessed pocket or through wall window 30, 32 that contains composite matrix filler 40, 38. FIG. 4 shows a cross-section of a typical hybrid airfoil design, illustrating a shallow pocket 30 that is filled with a composite or polymer material 40.

This hybrid blade design allows for several beneficial outcomes. It creates a lighter bucket which allows for longer or wider chord buckets. A longer bucket will allow for more steam flow, thereby increasing the turbine output. A lighter bucket also allows for wider chord buckets or buckets with improved aerodynamics, thereby in providing stage efficiency.

The hybrid bucket design also affords the ability to “mixed tune” the continuously coupled bucket stage to dampen the overall frequency response of the stage. Further, the hybrid bucket has the opportunity to reduce costs. The titanium currently used on the longest buckets that are produced is very costly, at up to 3× the cost of steel alloy. The hybrid bucket has the opportunity to replace titanium designs with a steel design with hybrid pocketing. There is also the opportunity of lengthening the useful life of the bucket stage by adding the hybrid bucket material thereby reducing stress levels in both the bucket and rotor. Additionally, one could arrange more than one stage with a hybrid design that would increase aeroefficiency or increase bucket length to produce more power. Even further, the hybrid bucket, being lighter allows for more flexibility in adjusting the IRD (inner hub or root diameter) of the bucket. Making the IRD larger for the same bucket allows for more annulus area should it be required in the thermodynamic/performance design. On a typical turbine moving the bucket outboard increases the pull load on the rotor significantly due to the exponential factor increasing the bucket pull load. Additionally, one could make a longer bucket while maintaining or reducing the IRD, both of which produce more annulus area. The new IGCC turbine design concepts require more annulus area due to the higher flow rates of that particular application. The larger hybrid bucket annulus area makes that possible without having to create more LP sections to pass the flow. This is not physically obtainable with current metallic buckets due to length (stress) limitations.

An objective of the invention is to produce a steam turbine system design to be used in conjunction with hybrid last stages LP buckets of the type generally described above. However, a couple of issues exist in making a hybrid system design achievable. One issue is that of the high temperature that is created during low VAN operation. As noted above, a significant issue in using hybrid bucket design, that is, composite or polymer material in a metallic blade, is the temperature condition during flow (low VAN) operation when the rotor is at full speed. During low flow operation the bucket tip region is in a windage condition that heats up the flow to significantly higher temperature than at steady state operation. Thus, the hybrid bucket system design must be able to overcome the temperature increase.

One way to make the hybrid bucket design feasible is to develop high temperature composite materials for use in a high temperature steam environment. See in this regard co-pending application Ser. No. 10/900,222, filed Jul. 28, 2004, the disclosure of which is incorporated herein by this reference. A second approach, as set forth in greater detail below, is to actively cool the bucket tip region during the low VAN windage condition.

In one example embodiment as illustrated in FIG. 5, the system incorporates water sprays 44 in the exhaust area that are selectively turned on when the low VAN windage condition exists. Such sprays may be optimized through testing or CFD analysis. The sprays are positioned either on the outer exhaust flow cone 46 or the inner cone 48, or both as illustrated in FIG. 5. The direction and amount of flow is desirably optimized such that there is enough water flow to cool the bucket 24 but not so much as to create excessive erosion of the bucket leading or trailing ends.

According to a further feature of the invention, which may be combined with water spray(s) 44 or provided in the alternative, is to inject steam or water from the outer side wall 50 of the last stage diaphragm 52 as illustrated in FIG. 6 or just forward of the bucket 24 tip as illustrated by way of example in FIG. 7. Yet a further alternative, using a configuration as in FIG. 8, is to provide a small extraction groove 54 in the nozzle outer side wall 50 near the bucket tip and just forward of the bucket 24, to extract flow, thereby reducing the windage heating condition.

Referring more particularly to FIG. 5, as illustrated therein the two final stages of the steam turbine are depicted as incorporating hybrid blades. In this example embodiment, upper and/or lower water sprays 44 are depicted in the low pressure exhaust hood diffuser area for cooling purposes. Additionally, or in the alternative, steam or water injection 56 is provided upstream of the nozzle diaphragm 52 (FIG. 6) or steam/water injection and/or steam rejection are provided on the downstream side of the nozzle diaphragm 52.

Referring more particularly to the steam or water injection options, FIG. 6 schematically illustrates a steam (or water) injection cavity 58 defined in the outer ring 50, including a steam (or water) injection port for directing steam 56 toward the upstream end of the last stage nozzles 60, said steam being conducted to the steam injection cavity through a steam injection port 62 in the low pressure casing. FIG. 6 also schematically illustrates water sprays 44 for cooling in the low pressure exhaust hood/diffuser area downstream of the last stage hybrid blades 24.

As noted above, steam or water injection and/or steam extraction may also be provided on the downstream side of the nozzle diaphragm 52, upstream of the hybrid blades 24. Thus, as illustrated in FIG. 7, main steam 64 may be directed through a steam injection cavity 66, passage 68 and groove 70, to be directed for cooling the downstream end of the nozzles 60 and the tips of the adjacent hybrid bucket 24. This injection flow helps reduce the temperature in the area that is prone to the windage heating condition. In the alternative, the steam injection groove 170 may be configured as a scoop 54, as illustrated more specifically in FIG. 8, whereby as an alternative to steam injection, moisture extraction can be accomplished through the moisture extraction scoop 54, extraction hole(s) or passage(s) 168 into the moisture extraction cavity 166 and ultimately e.g., to the condenser. Using a small extraction groove just forward of the bucket to extract flow reduces the windage heating condition.

As illustrated in FIG. 5, the hybrid bucket system could involve more than one stage, particularly as higher temperature composite materials are developed. This would allow for increased area or increased efficiency airfoils in the last stage(s). Additionally, this would allow for smaller and shorter rotor wheels due to the reduced pull load of the buckets. The shorter rotor would allow for less plant space. A shorter rotor could also benefit the rotor dynamics as more critical fundamental frequencies many move above running speed. In addition, the hybrid bucket system design would allow for a larger annulus by moving the same length bucket outboard. This would not be possible using a conventional rotor design as the bucket would weigh too much for the rotor to be able to handle the increased loading. This allows more turbine output with the same bucket length.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 

1. An axial flow steam turbine including: a rotor; a last stage including a diaphragm comprising an inner ring, an outer ring, and a plurality of nozzles extending therebetween, and a row of buckets secured to said rotor downstream of said diaphragm for rotation with respect to said last stage diaphragm; and at least one injection assembly for injecting cooling media toward a vicinity of said last stage.
 2. An axial flow steam turbine as in claim 1, wherein the injection assembly comprises water sprays in an exhaust area downstream of the last stage buckets.
 3. An axial flow steam turbine as in claim 1, wherein said injection assembly comprises a steam or water injection cavity defined in said outer ring and a steam or water injection opening directed towards said nozzles of said diaphragm.
 4. An axial flow steam turbine as in claim 3, wherein said injection opening is disposed upstream of the nozzles with respect to a steam flow path through the last stage.
 5. An axial flow steam turbine as in claim 3, wherein the injection cavity is downstream of the nozzles with respect to a steam flow path through the last stage.
 6. An axial flow steam turbine as in claim 1, comprising a moisture extraction assembly for extracting moisture from a steam flow path through the last stage, downstream of the nozzles and upstream of the buckets with respect to said steam flow path.
 7. An axial flow steam turbine as in claim 6, wherein moisture extraction assembly comprises a moisture extraction cavity defined in said outer ring and a moisture extraction groove having an opening having a scoop directed towards said nozzles of said diaphragm.
 8. An axial flow steam turbine as in claim 1, wherein at least one said bucket comprises a) a shank portion; and b) an airfoil portion having an operating temperature range, a design rotational speed, a blade root attached to said shank portion, a blade tip, and a radial axis extending outward toward said blade tip and inward toward said blade root, and wherein said airfoil portion also includes: (1) a metallic section consisting essentially of metal and having a first mass density, wherein said metallic section radially extends from generally said blade root to generally said blade tip; and (2) at least one fiber composite section, having a second mass density less than said first mass density.
 9. An axial flow steam turbine as in claim 8, wherein said metallic section and said at least one fiber composite section together define a generally airfoil shape at said design rotational speed.
 10. An axial flow steam turbine as in claim 8, wherein said fiber composite section is disposed in a pocket defined in a pressure side of said metallic section.
 11. In an axial flow steam turbine including a rotor and a last stage including a diaphragm comprising an inner ring, an outer ring, and a plurality of nozzles extending therebetween, and a row of buckets secured to said rotor downstream of said diaphragm for rotation with respect to said last stage diaphragm, a method of cooling said last stage, comprising: injecting cooling media toward a vicinity of said last stage.
 12. A method as in claim 11, wherein said injecting comprises spraying water in an exhaust area downstream of the last stage buckets.
 13. A method as in claim 11, wherein said injection comprises injecting steam or water from a steam or water injection cavity defined in said outer ring through a steam or water injection opening directed towards said nozzles of said diaphragm.
 14. A method as in claim 13, wherein said injection opening is disposed upstream of the nozzles with respect to a steam flow path through the last stage.
 15. A method as in claim 13, wherein said injection opening is disposed downstream of the nozzles with respect to a steam flow path through the last stage.
 16. A method as in claim 11, further comprising extracting moisture from a steam flow path through the last stage, downstream of the nozzles and upstream of the buckets with respect to said steam flow path.
 17. A method as in claim 16, wherein moisture is extracted through a moisture extraction groove having an opening including a scoop directed towards said nozzles of said diaphragm, and into a moisture extraction cavity defined in said outer ring.
 18. A method as in claim 11, wherein at least one said bucket comprises a) a shank portion; and b) an airfoil portion having an operating temperature range, a design rotational speed, a blade root attached to said shank portion, a blade tip, and a radial axis extending outward toward said blade tip and inward toward said blade root, and wherein said airfoil portion also includes: (1) a metallic section consisting essentially of metal and having a first mass density, wherein said metallic section radially extends from generally said blade root to generally said blade tip; and (2) at least one fiber composite section, having a second mass density less than said first mass density.
 19. A method as in claim 18, wherein said metallic section and said at least one fiber composite section together define a generally airfoil shape at said design rotational speed.
 20. A method as in claim 18, wherein said fiber composite section is disposed in a pocket defined in a pressure side of said metallic section. 